A semishell-shaped sheet gripper of this general type is known from DE 3,623,405 C2. The gripper consists of a box-shaped gripper finger and a carrier whose respective ends, adjacent the transfer point, are spread apart by means of a compression spring. An adjusting screw is arranged at the opposite end to regulate the spacing. The carrier is fastened by means of a screw on the gripper actuating shaft which also carries the gripper finger. Due to the box shape, the gripper finger forms an integral peripheral surface which constitutes an effective bearing surface on the gripper shaft. This gripper apparatus is easy to assemble, but has an unfavorable gripper finger geometry. The gripper finger encounters play on the gripper shaft resulting in a tendency of the gripper finger and, thus the gripper tip, to slide due to the position of the gripper tip in the triangle of effective forces. Due to the peripheral surface interacting with the gripper shaft, a high break-away moment is produced during opening and closing which also contributes to the sliding of the gripper tip during closing.
DD-WP 101 353 also discloses a semishell-shaped sheet gripper which has a guide groove between the gripper spindle and the gripper finger paired with a guide ring or guide ball. Additionally, two springs are positioned opposite each other. The disadvantage of this gripper arrangement is high manufacturing costs as well as unreliable lateral guiding of the gripper finger.
DE-C-2,725,035 discloses a sheet gripper having a special gripper shaft contour. The disadvantage of this disclosure is that it is very expensive to manufacture the gripper shaft. Moreover, the holding force is difficult to adjust.
The design of a rounded gripper tip is known from DE 3,739,169 C1, which has a bead with a radius of 0.4 to 1.0 mm, which is coated with a mechanically resistant material and runs parallel to the leading edge of the sheet. It is disadvantageous in that a high surface pressure occurs due to the bead-like elevation as a result of the linear contact with the printing carrier during closing. This surface pressure leaves impressions or imprints in the printing carrier and the gripper tip tends to displace the printing carrier, especially when relatively thin paper is used.